had the same thing happen with all fruit trees i grew in ground in my clay soil. it killed every one of them. its drainage issues. i bet if you grabbed that tree, you could easily pull it out. the roots are rotting. i had to grow mine in mounds or raised beds to get them to flourish. all of them got that dark sunken bark. unfortunately, i dont think you can do much other than replant. are the leaves off colored and weak looking? it took 2 seasons for them to finally die.
maybe they are just in a bad spot in the yard? my father had a spot like that. was pure gray clay. he tried amending but always went back to pure clay. never grew well. he wound up making a raised bed over it. my yards a mix of gray shale clay and red clay with lots of rocks about fist sized. bushes and native trees grow fine in it but anything that needs well drained gets killed eventually.
At times the soil is too heavy ( clay) and they get water logged. I had one that never grew and looked like that. I took it out and the roots never developed. The main bigger root was there but no smaller roots or hair roots ever developed.
I put another tree in that spot and put some peat moss mixed with the dirt I took out of the hole and the next tree did fine. Not saying that is your answer it is just what I did that seemed to work.
I also like the idea of a raised bed area to plant your tree in that steveb4 mentioned. I have a tree I need to do that with. There are some roots growing above the soil and I need to do that so the roots do not dry out. I am not sure why the heck this is happening but it is a different rootstock that what I normally use. It is Support 4 rootstock. I had never heard of it before nor knew anyone else that had used it. It seems okay except for the burr knot looking roots that are growing above the soil. Odd.
There is a possibility that you have a clay hardpan that prevents free drainage. Try digging down to several feet to see if you can get thru the rocklike hardpan. I use a pickax here in my yard and often I can pick through the hardpan in places allowing better drainage. Also if you are successful, getting through the hardpan, fill the hole with water and see if it drains within 24 hours. If so then try to find some organics to mix in with the native soil. About 50/50 mix should work. If the hole does not freely drain, then you know you have to create growth mounds that drain well.